Digitizing of analog signals has long been used to provide an efficient basis on which the signals can be manipulated and analysed. In the medical field, for example, the outputs of nuclear or scintillation cameras have been digitized so that their images may be analyzed and displayed through a digital computer. Such nuclear imaging typically produces a single frame constructed over a long period of time (i.e., measured in minutes rather than in microseconds), and thus the digitization of these slowly forming images did not involve the problems inherent in digitizing high speed sequential imaging. With the advent of ultrasound radiographic and other types of video imaging, it has been possible to produce an output video signal employing a raster scan to show successive frames of an event as it occurs or, as it is commonly referred to in the art, "in real time". These video signals are typically recorded on videotape for later playback. Because the raster scan of a standard video signal produces 30 (or 25 depending on the television system employed) complete frames per second, the digitization of such a signal involves entirely new problems not encountered in single frame nuclear imaging.
Digitization of a raster scan requires both an extremely large memory system and the hardware capable of digitizing and storing this data at a very high rate of speed. Furthermore, if such a system is to be most effective in its intended environment, the hardware must be relatively portable so that it may be moved proximate the subject to be tested (such as at a patient's bedside). Because of these environmental requirements, it is necessary to provide hardware which can reduce memory requirements, and thus reduce the physical size of the memory required, by selecting the most pertinent data to be digitized or by compressing adjacent bits of data (pixels) into fewer number. With these schemes for better utilizing available memory space, it is possible to store a large number of sequential images in digital form.